1. Technical Field
This disclosure generally relates to earth boring bits used to drill a borehole for applications including the recovery of oil, gas or minerals, mining, blast holes, water wells and construction projects. More particularly, the disclosure relates to percussion hammer drill bits.
2. Description of the Related Art
In percussion hammer drilling operations, the bit impacts the earth in a cyclic fashion while simultaneously rotating. In such operations, the mechanism for penetrating the earth is of an impacting nature rather than shearing. Therefore, in order to promote efficient penetration by the bit, the cutting elements of the bit need to be “indexed” to fresh earthen formations between each impact. This need is achieved by rotating the drill string a slight amount between each impact of the bit to the earth and incorporating longitudinal splines which key the bit body to a cylindrical sleeve (commonly known as the driver sub or chuck) at the bottom of the hammer assembly. As a result of this arrangement, the drill string rotation is thereby transferred to the hammer bit itself. Experience has demonstrated for an eight inch diameter hammer bit that a rotational speed of approximately 20 rpm for an impact frequency of 1600 bpm (beats per minute) results in efficient drilling operations. This rotational speed translates to an angular displacement of approximately 4 to 5 degrees per impact of the bit against the rock formation.
An example of a typical hammer bit connected to a rotatable drill string is described in U.S. Pat. No. 4,932,483, incorporated herein by reference. The downhole hammer comprises a top sub and a drill bit separated by a tubular housing incorporating a piston chamber therebetween. A feed tube is mounted to the top sub and extends concentrically into the piston chamber. A piston is slideably received within the housing and over the feed tube. Fluid porting is provided in the feed tube and the piston. This porting admits fluid in a first space between the piston and top sub to drive the piston towards the drill bit support, and thereafter to a second space between the piston and the drill bit support to drive the piston towards the top sub.
Rotary motion is provided to this conventional hammer assembly and drill bit by the attached drill string which, in turn, is powered by a rotary table typically mounted on the rig platform or by a top drive head mounted on the derrick. The drill bit is rotated through engagement of a series of splines on the bit and driver sub that allow axial sliding between the two components.
Due to the forces transmitted between the splines, as well as the cyclic nature of the stress created, mechanical failure of the splines can force an operator to remove the drill bit from operation for repair or replacement, thereby increasing maintenance and operation costs. If a portion of the drill bit completely fractures, it can become separated from the rest of the percussion drill assembly. In such a case, mere removal of the drill assembly from the borehole by withdrawing (or “tripping”) the drillstring will not extract the fractured portion of the drill bit. Instead, the fragment must be removed by a separate and time-consuming procedure, adding still further cost. It is therefore desirable to retain any fractured portions of the drill bit with the rest of the percussion drill assembly, thereby allowing the fractured portion to be extracted simultaneously with the withdrawal of the drillstring from the borehole.
The embodiments of the present invention described herein provide opportunities for improvement in retaining the drill bit in the event of a fracture. These and various other characteristics and advantages will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.